1. Field of the Invention
This invention relates generally to systems for transmitting digital data over telephone lines and, more specifically, to such systems which automatically compensate for distortion of the transmitted signal by the telephone line.
2. Description of the Prior Art
With the advent of large scale digital data processing systems is has become increasingly desirable to transmit such data over large distances with a high degree of accuracy. For example, a retail store chain might have a central warehouse facility with which each of the stores might advantageously communicate to maintain its respective inventory. A branch bank might also find it desirable to transmit accounting data to a central accounting facility.
Particularly in the latter example, the accuracy of the transmission is of paramount importance. To enhance the accuracy of digital transmissions, the data could have been transmitted in blocks of rows and columns each having a parity bit which indicates whether the data in that row or column was accurately received. Ideally, when the parity bits indicate there has been an error in transmission, the receiver requests that the data be retransmitted. This request is preferably made through a secondary channel so that the receiver can communicate with the transmitter as the data is being transmitted. This can substantially reduce the time needed for transmission since each erroneous block of data can be immediately retransmitted.
It has been desirable to transmit the data over telephone lines because of their availability and wide distribution. However, a telephone line is well known to have a passband which at best is limited and may be even further degraded depending upon the age and quality of the line. For this reason, data transmission systems of the prior art, which have generally required relatively wide passbands, have typically occupied the entire passband of the telephone line. It follows that those telephone lines having significantly degraded passbands have been unacceptable for this type of transmission. These telephone lines of poor quality have been particularly apparent where a system has used a portion of the telephone passband to accommodate a secondary channel for system control.
Due primarily to the wide variation in the passbands of the dial-up type of telephone lines, such lines have generally not been used for the transmission of digital data. Rather, telephone lines have typically been leased in order to obtain a guarantee of the passband quality. Leased telephone lines have not been satisfactory for a number of reasons. First, leased lines are very expensive. Second, leased lines are generally not used 100% of the time so that often the high cost must be prorated over a relatively sort period of use. Third, it has been more difficult to maintain the confidentiality of data transmitted on a leased line since such a line is more vulnerable to being tapped. This is of particular importance to a bank which must insure the secrecy of its records.
Any system for transmitting data over a telephone line must overcome other problems which also result primarily from the difference in quality of the transmission lines. A most significant problem has been the considerable delay and attenuation distortion of the transmitted signal by the telephone line. Due to this distortion, signal components at certain frequencies within the audio passband are delayed and attenuated to a greater extent by the telephone lines than are signal components at other frequencies. Although this delay and attenuation distortion does not significantly impair the intelligibility of voice signals, it does cause severe distortion of digital signals transmitted over these lines.
The telephone lines have also produced rapid variations in the difference between the phase of the modulation carrier of the transmitter and the phase of the demodulation carrier of the receiver. This variation is commonly referred to as phase jitter. The transmission lines have also produced frequency offset wherein the whole spectrum of the transmitted signal is shifted.
The attempts of the prior art systems to solve this problem for transmission of 4800 bits per second (bps) over dial-up telephone channels have not been entirely satisfactory. For example, the prior art systems have included automatically equalized 4800 bps modems which have been used, in general, either two-level partial response single sideband amplitude modulation, or four-level straight quadrature double sideband suppressed carrier amplitude modulation. These systems typically have had a bandwidth of at least 2400 Hertz for the primary channel. Telephone lines at best have a passband of 2400 Hertz so that lines of poor quality have not been capable of accommodating a spectrum of this width. As a result, the systems of the prior art generally have only been operable over the best telephone lines. Also, due to the relatively wide bandwidth of the primary channel, it has been difficult to provide the systems of the prior art with secondary channels for system control.
In the past, a number of techniques have been used to correct or equalize the transmission line distortion. In one system, the distortion of the transmission line is determined and, prior to transmission, the data is predistorted in such a way that the additional line distortion alters the transmitted signal to produce an undistorted received signal. This system is particularly tedious, and its use is clearly limited to those situations where the delay and attenutation characteristics of the line are constant and known.
Other transmission systems have been designed to manually compensate for the unknown characteristics of the transmission line at the receiver. After measurement of the line characteristics, these networks have been manually adjusted to provide additional delay and attenuation characteristics for those frequencies which experience the least delay and attentuation over the transmission line. While widely used, these equalization systems suffer a considerable disadvantage in that they have to be manually adjusted each time a change in line characteristics occurs. These adjustments are both tedious and time consuming.
Still a further technique for correcting the distortion of the transmitted signal involves the use of decision feedback equalization. In such a technique, the detected data samples are cross-correlated with the received signal to obtain samples of the impulse response of the channel. Then the previously detected data samples are multiplied with the impulse response samples and subtracted from the incoming signal to eliminate intersymbol interference. This cross-correlation, however, is only responsive to intersymbol interference which follows a signaling pulse. In general, the intersymbol interference leads as well as follows the signaling pulse, so that the decision feedback removes only a portion of the intersymbol interference. Furthermore, if an error is made in detecting the data, the erroneous data pulse is multiplied by the impulse response and, instead of subtracting the intersymbol interference, it actually adds to it. This avalanche of errors has been a significant drawback in these feedback or recursive type equilization systems.
Automatically equalized modems using partial response signalling in a single sideband system have typically used the product of the sign of the unequalized signal and the error signal to provide equalizer tap adjustments. This technique has relied upon the provision of sufficient circuitry not only to determine but also to store the sign of the unequalized signal.
In the past, equilization and lowpass filter circuits, which have typically followed a single sideband coherent demodulation circuit, have introduced considerable delays into the system. A phase error signal, which has typically been produced by a circuit following the equalization circuit, has been used to drive a voltage controlled oscillator in the coherent demodulation circuit. In this manner, a phase lock loop including the equalization and lowpass filter circuits has been provided. Unfortunately, the delays associated with these two circuits in the loop have provided the phase correction signal with a low frequency response which has been relatively incapable of tracking rapid phase jitter.
Timing recovery and control has been provided in quadrature receivers including consecutively samplers, coherent demodulators, lowpass filters, equalizers and detectors. The signals at the outputs of the lowpass filters have formed an eye pattern having a plurality of zero crossings. To provide accurate detection of the signals it has been desirable to time the sampler so that samples are taken at times corresponding to these zero crossings of the eye pattern. In accordance with the timing methods of the prior art these zero crossings have been detected and the rate of the sampler has been adjusted accordingly. This timing procedure has not been particularly accurate since the eye pattern at the output of the lowpass filters has not been well defined due to intersymbol interference which has been corrected subsequently. This eye pattern would be even further adversely effected if the phase of the signal were not connected previously, as in the case of non-coherent demodulation.
No data transmission system of the prior art has combined all the desirable features of a transversal equalizer, a single high-frequency phase jitter and frequency translation phase lock loop, an effective time loop for synchronizing the receiver with the transmitter, and (1,1) partial response signaling.